On December 28, 1847 the Provisional Legislature created Linn County from the southern portion of Champoeg (later Marion) County. The boundaries were altered in 1851 and 1854 with the creation of Lane and Wasco Counties. The county seat was originally located in Calapooia (later known as Brownsville), but in 1851 the Territorial Legislature passed an act establishing Albany as the county seat. A special election in 1856 reaffirmed Albany as the county seat.

There were 39,541 households out of which 32.00% had children under the age of 18 living with them, 56.90% were married couples living together, 10.00% had a female householder with no husband present, and 28.60% were non-families. 23.00% of all households were made up of individuals and 10.10% had someone living alone who was 65 years of age or older. The average household size was 2.58 and the average family size was 3.01.

In the county, the population was spread out with 26.00% under the age of 18, 8.40% from 18 to 24, 27.00% from 25 to 44, 24.10% from 45 to 64, and 14.50% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 97.50 males. For every 100 females age 18 and over, there were 95.00 males.

The median income for a household in the county was $37,518, and the median income for a family was $44,188. Males had a median income of $35,586 versus $24,073 for females. The per capita income for the county was $17,633. About 8.90% of families and 11.40% of the population were below the poverty line, including 14.80% of those under age 18 and 7.10% of those age 65 or over.

In January 2013, Linn County Sheriff Tim Mueller gained national attention for a letter he sent to Vice-President Joe Biden, informing Mr. Biden that he [Mueller] would not enforce any federal firearms laws he considered to be "offending the constitutional rights of my citizens." He further stated that he would not permit federal officers to come to his county to enforce such laws.[11] A copy of the letter may be seen on the Linn County Sheriff's Department Facebook page.[12]

Principal industries are wood products, agriculture, mining, and manufacturing. Linn County's economy relies heavily on the lumber and wood products industry; in 1990, this industry accounted for 40% of the county's manufacturing jobs. The climate and soil conditions provide one of Oregon's most diversified agriculture areas, allowing a wide variety of specialty crops such as common and perennial ryegrass. Linn County is also home to the only emery mine in the United States, and the production of manufactured and motor homes.

A variety of flora and fauna occur in Linn County. Fauna include mammals, birds, amphibians and reptiles. Trees include a variety of oaks and conifers as well as other species such as Salix sessilifolia.[13] The Rough-skinned Newt is a common amphibian occurring in the oak woodlands of the county.[14]Carpenter Mountain, one of the highest points in the western Cascades is located in Linn County, although road access to the peak is only possible through Lane County.